1. Field of the Invention
The apparatus of the present invention relates generally to devices for controlling the mode of operation of an electronic musical instrument, and more particularly to a touch operated capacitive switch for altering electrical signals controlling the mode of operation of an electronic musical instrument and having means for increasing the sensitivity of the switch.
2. Description of the Prior Art
Devices for changing the mode of operation of an electronic musical instrument upon command by a performer at any time during a musical performance are well known in the art. For example, individual mechanical switches have long been used with electronic keyboard instruments, such as organs and the like, to provide pleasing variations in pitch, timbre and rhythm, among others, to add interesting variation and color to a musical performance. However, heretofore it has been difficult for a performer to actuate such devices during a performance without interruption to the musical work being performed, such as occurs when the performer must remove his hands from a playing position on an instrument keyboard, for example.
One arrangement which has been proposed for overcoming this problem includes a touch actuated switching device mounted on the musical instrument in such a position that it can be activated by the performer's hands while in their playing position. In one proposal, the touch actuated switching device takes the form of an elongated pressure or diaphragm switch mounted on the keyslip of a keyboard instrument such that the keyboard and the switch may be touched simultaneously by the performer. Thus, when the performer wishes to alter a particular mode of operation of the instrument, he need merely press the switch at any point along its length with one or more fingers. It has been found, however, that this type of switch requires considerable applied pressure for its activation, placing an unnecessary burden on the performer to apply the proper pressure and insure that the switch has in fact been actuated. In addition, since the switch is mechanical in nature it is subject to possible mechanical wear and failure, resulting in increased operating and replacement costs, as well as reduced reliability.
It has also been suggested to use a touch operated capacitive switch of limited active area to permit change in the mode of operation of the musical instrument. The operation of such switches is well understood by those skilled in the art and need only be briefly described herein. In its usual form, the touch operated capacitive switch comprises a small conducting plate impressed with a small alternating voltage. The plate of the switch is so positioned and constructed that in its quiescent state it possesses minimal intrinsic capacitance. Hence, the alternating voltage applied to the plate of the capacitive switch produces a negligible quiescent current flow.
However, when the plate of the switch is touched, such as by the extended finger of a musical performer, the capacitance associated with the performer's body existing between his finger and ground, causes an increased current to flow from the plate of the switch. This increased current can be detected by any one of a number of conventional and well understood current detection circuits to provide a change of electrical state for changing the operational mode of the musical instrument as desired. It has been found, however, that the inherent capacitance associated with the human body is relatively small, producing a correspondingly small current change when the switch plate is touched. Hence, if the surface area of the switch plate is permitted to become too large, the quiescent current associated with the large plate may completely swamp a small current change caused by touching the plate, making detection of the latter current impossible. For this reason, capacitive touch switches associated with electronic musical instruments have heretofore been of relatively small size, and have been mounted on the instrument in a location preventing the performer from activating the switch without interrupting his performance. Consequently, capacitive touch switches spanning a substantial part of the length of a musical instrument keyboard, for example, have heretofore proved impractical.
Attempts have been made by prior art workers to neutralize the intrinsic capacitance associated with large capacitive touch switches, but such attempts have proved largely unsuccessful. For example, one method previously employed to neutralize the effect of the large capacitance has been to provide a current offset exactly equal to the quiescent current caused by the large plate capacitance, such as through a bridge network or constant current source. Having balanced out the quiescent current in this way, small changes in current caused by touching the plate of the switch are easily detected. It has been found, however, that the adjustment necessary to null the quiescent current is extremely critical; in addition, small changes in mechanical dimensions of the plate of the touch switch, such as might be caused by vibration or thermal expansion or contraction with changes in temperature, will unbalance the circuit, causing a significant decrease in sensitivity of the switch.
Attempts have also been made to reduce the intrinsic capacitance of the switch plate by locating the switch on the instrument at a place removed from points of ground potential. However, in some applications, for example where the touch switch is required to be mounted on a metallic keyslip adjacent a keyboard, it has proved impractical to reduce the intrinsic capacitance of the switch in this manner. Likewise, attempts to electrostatically shield the switch plate have also proved largely unsuccessful.